Credit Suisse admitted Thursday that it was under investigation by U.S. authorities for its use of "dark pools", becoming the latest in a string of European banks to be probed for use of the controversial alternative trading platform.
Earlier this week, Germany's Deutsche Bank and Switzerland's UBS said they were being probed by U.S. regulators, who are looking into whether their platforms gave an unfair advantage to high-frequency traders.
Meanwhile, Barclays has urged the dismissal of a U.S. investigation into allegations it misled clients about its dark pool, while Goldman Sachs has paid $800,000 to settle claims it failed to ensure trades on its platform took place at the best possible price. Goldman Sachs didn't admit nor deny the charges.
Here's a quick look at the dark pool trading platforms and why it's come under investigation.
What are 'dark pools'?
"Dark pools"—also known as "black pools", "dark liquidity" and "upstairs markets"—are private forums for trading securities which are not available to the public. The names refer to the trading venues' lack of transparency.
The trading platforms allow investors to make large trades anonymously and prices are only published after trades have been made. This reduces the chance of information leaking about trade orders and allows firms to avoid paying exchange fees.